Shaping a lens to enable it to be mounted in or on a frame selected by the future wearer consists in modifying the outline of the lens so as to adapt it to the frame and/or to the shape desired for the lens. Shaping the lens includes edging in order to shape the periphery of the lens, and, depending on whether the frame is of the rimmed type (the frame having rims presenting an internal bezel forming a groove), of the drilled type (with a rimless frame and point connections through fixing holes formed in the lens), or of the grooved type (with a frame possessing firstly two half-rims each presenting a bevel or a bezel as in rimmed frames, and secondly a nylon string passing around the remainder of the outline of the lenses), shaping also involves appropriately beveling or grooving the lens, and/or drilling it. With a drilled type frame, after being shaped, the lens is drilled at fastener points for the nose bridge either using the same shaper device or else using a separate appliance.
Edging proper consists in eliminating the superfluous peripheral portion of the ophthalmic lens in question so as to transform its initial outline, which is usually circular, to the outline desired for the rim of the frame of the eyeglasses in question, or merely to the desired shape when the frame is of the rimless type. This edging operation is usually followed by a chamfering operation which consists in rounding or chamfering the two sharp edges at the edge of the edge lens. When the frame is of the rimmed type, this chamfering is accompanied or preceded by a beveling operation which consists in forming a rib usually called a bevel and generally of triangular cross-section on the edge face of the ophthalmic lens. This bevel is designed to be engaged in a corresponding groove, commonly referred to as a bezel, formed in the rim of the frame in which the lens is to be mounted. When the frame is of the rimless type, the operations of shaping the lens and optionally rounding its sharp edges (chamfering) are followed by appropriately drilling the lenses so as to enable the branches (temples) and the nose bridge of a rimless frame to be fastened. Finally, when the frame is of the type that has a nylon string, chamfering is accompanied by grooving which consists in forming a groove in the edge face of the lens, this groove serving to receive the nylon string of the frame for pressing the lens against the rigid portion of the frame.
Conventionally, such shaper means are constituted by a machine tool referred to a grinder that possesses a set of main grindwheels and means for blocking and imparting rotary drive to the lens, which means are constituted by two rotary shafts lying on the same axis and mounted to move relative to each other in an axial direction in order to clamp the lens on said axis between them. In order to enable the lens to be moved towards or away from the grindwheels during machining, the clamping and drive shafts are carried by a rocker that is movable (in pivoting or translation) transversely relative to the shafts.
As a general rule, the operations of shaping, chamfering, and beveling are performed in succession on a single grinder that is fitted with a suitable set of main grindwheels. Drilling, when required, can be performed on the same grinder, which then needs to be fitted with corresponding tooling, or else on a distinct drilling machine.
The optician needs also to perform a certain number of measurement and/or identification operations on the lens itself, prior to shaping, in order to identify certain characteristics of the lens such as, for example: its optical center if it is a single vision lens, or the mounting cross if it is a progressive lens, or the direction of the progression axis and the position of the centering point of a progressive lens.
In practice, each lens is generally delivered by the manufacturer with marks on its concave front face, some of which marks identify a centering frame of reference for the lens. If these marks on the ophthalmic lens themselves are not sufficiently visible, the optician marks certain characteristic points using a marker tip. These marks are used for positioning and fastening an adapter or centering-and-drive pad on the lens so as to enable the ophthalmic lens to be positioned properly in the machine tool that is to give it the desired outline corresponding to the shape of the selected frame. The operation of positioning and depositing the pad can be performed manually or automatically, using an appliance referred to as a centering and blocking device.
In any event, the pad is usually stuck temporarily on the lens with the help of a double-sided adhesive. This operation is conventionally referred to as centering the lens, or by extension blocking the lens, insofar as the pad enables the lens subsequently to be blocked, i.e. prevented from moving, on the means for shaping it and in a geometrical configuration that is known by virtue of the pad.
After the centering pad has been put into place, the lens fitted therewith is subsequently placed in the shaper machine where it is given the shape that corresponds to the shape for the selected frame. The centering pad serves to define and to physically employ on the lens a geometrical frame of reference in which characteristic points and directions of the lens are identified together with shaping values, as are needed for making the lens coincide with the position of the pupil, so as to ensure that these characteristic points and directions are properly positioned in the frame.
When the first attempt at shaping the lens does not succeed in enabling it to be properly mounted in the frame, the operator restarts machining. To do this, the lens is put back in the machine and is blocked using the same pad, thus enabling the initial frame of reference used for shaping to be recovered.
Nevertheless, the use of a stuck-on pad constitutes a drawback insofar as the pad needs to be removed after the lens has been mounted, thereby consuming time and labor. In addition, the lens is secured to the pad by adhesive, which can require intensive cleaning of the surface of the lens after the treatment, running the risk of scratches. Finally, since these operations of placing and removing the pad are relatively complex and difficult, they must be performed by qualified and careful personnel, which in practice consumes a large amount of time and is thus expensive; for the same reasons, these operations turn out to be difficult to automate.
Thus, in the context of its research work, the Applicant is seeking to avoid centering by means of a pad because of the above-mentioned constraints.
However, under such circumstances, in which a pad is no longer put into place prior to the first machining operation, the lens is centered and blocked on the clamping-and-drive shafts by optical measurement means and/or mechanical handler means. Optical measurements provide a theoretical centering frame of reference for the ophthalmic lens relative to the clamping shafts. Inaccuracies in centering and blocking the lens, and also in the measurement and handler means, have the effect that a first real frame of reference is obtained for the lens relative to the clamping shafts that is slightly different from the theoretical frame of reference calculated from the optical measurements. The first machining operation is performed in this first real frame of reference.
The lens is then shaped by machining using cylindrical roughing-out and finishing grindwheels whose shaping faces are parallel to the axis of rotation of the clamping-and-drive shafts, said grindwheels forming part of a main grindwheel set and being mounted to rotate about the axis of rotation of the grindwheel set.
After the first machining operation, the lens is unblocked, and is therefore separated from the blocking chucks of the clamping shafts. As a result of this unblocking, the first real centering frame of reference is lost.
When previous shaping of the lens in a first machining operation does not produce the desired result, the optician needs to restart shaping in a second machining operation.
In order to restart machining correctly, the lens ought to be placed in the real centering frame of reference that was used during the first machining operation so that the edging face of the working grindwheel is indeed parallel to the edge face of the lens for reworking.
Prior to the second machining operation, optical measurements are used to recalculate the theoretical centering frame of reference for the lens. Inaccuracies in these optical measurements mean that the real centering frame of reference obtained in the second machining step differs slightly from the theoretical first frame of reference used during the first machining step. Furthermore, these optical measurement inaccuracies are in addition to inaccuracies in blocking the lens by the blocking chucks on the clamping shafts. The second real centering frame of reference that is actually obtained is thus different from the first in which it would be desirable for the lens to be replaced for reworking. This leads to an error in the positioning of the lens relative to the grindwheel during this second machining operation. In particular, the lens is off-center relative to its center position during the first machining operation, so the edge face of the lens is inclined relative to the edging face of the working grindwheel. Thus, machining in this configuration cannot obtain the desired radii of curvature in the edge face of the lens.
Furthermore, if the lens includes a bezel, the error in the positioning of the lens relative to the grindwheel means that when restarting machining the edging face of the grindwheel pares away the bezel in non-symmetrical manner.
The problem thus lies in restarting edging in the new centering frame of reference of the ophthalmic lens for eyeglasses in such a manner as to enable the edge face of the lens to be machined again correctly.
Document FR 2 811 599 describes a chamfering tool for improving the accuracy of a chamfering operation applied to a lens for eyeglasses. However that invention neither poses nor solves the technical problem of restarting edging in the new centering frame of reference of the lens.
It proposes inserting compensation means having the capacity to deform elastically between firstly the periphery in question of one or other of the elements constituting the chamfering tool used and the eyeglasses lens being worked, and secondly the support shaft for the same element.
However nothing is said concerning the use of such a tool for restarting edging of the edge face of an ophthalmic lens. The structural characteristics of the tool described do not lend themselves to such transposition. The chamfering tool does not have a face for edging the edge face of the lens.
In addition, the tool does not satisfy accuracy requirements for restarting edging the edge face of the lens and it cannot satisfy those requirements since the inserted compensation means leave the chamfering tool free to deform radially.